1. Field of the Invention
The present invention relates to reflectors which are suitable for use in liquid crystal displays using external light, a front light, a backlight, etc., as a light source, and to liquid crystal displays using the reflectors. More specifically, the present invention relates to a reflector which exhibits good reflectance over a wide angle range and especially high reflectance in a reflection direction in a desired range, and to a liquid crystal display which uses the reflector so that it has a wide viewing angle and exhibits moderate directionality such that the display appears sufficiently bright when seen from a typical viewing area.
2. Description of the Related Art
Liquid crystal displays are commonly used as display units for mobile computers, etc., and reflective liquid crystal displays, which use external light as a light source, are one kind of liquid crystal displays which are commonly used because of their low power consumption. In addition, liquid crystal displays having a front light for obtaining extra light in addition to external light are also commonly used.
In such liquid crystal displays, external light incident on the display surface (from the observer side) or light emitted from a front light is reflected by a reflector and is emitted outside the display surface, so that a user can view an image which changes in accordance with the alignments of liquid crystal molecules in a liquid crystal layer.
In addition, liquid crystal displays having a backlight for obtaining extra light in addition to external light are also commonly used. In liquid crystal displays having a backlight, a semi-transmissive reflector is used in order to reflect external light and to pass light emitted from the backlight.
The inventors have performed various investigations with respect to the relationship between the shape of the surface of a reflector (the shape of the surface closer to a display surface) and reflection characteristics of the reflector.
When a reflector having a flat, specular surface is used, the reflector exhibits extremely high reflectance at a specific reflection angle determined in accordance with an incidence angle. However, a reflection-angle range in which high reflectance is obtained is extremely narrow. Thus, a reflector having high directionality such that a viewing area from which the reflector appears bright is narrow is obtained. In addition, visibility is degraded due to so-called back reflection, that is, reflection of a light source, an observer's face, etc. in a display surface.
Accordingly, several techniques have been suggested in which concave portions having shapes like parts of a sphere, grooves, or irregular concavities and convexities are formed over the surface of a reflector in order to obtain good reflectance over a wide range. According to these techniques, reflection characteristics can be made such that the reflector appears bright over a wide viewing area.
FIG. 9 shows a reflector in which a plurality of concave portions each shaped like a part of a sphere are formed on the surface of the reflector. With reference to FIG. 9, a reflector 51 is constructed by forming a plate-shaped resin base member 53 (a base member of the reflector) made of a photosensitive resin layer, etc., on a substrate 52 made of glass, etc., and forming a plurality of concave portions 54 over the surface of the resin base member 53. The inner surfaces of the concave portions 54 are shaped like a part of a sphere, and the concave portions 54 are formed continuously so that the concave portions 54 overlap one another. In addition, a reflective film 55 formed of a thin layer of aluminum, silver, etc. is formed on the concave portions 54 by vapor deposition, plating, etc.
The concave portions 54 are formed such that the depth thereof varies in the range of 0.1 μm to 3 μm, and are irregularly arranged such that the pitch between the concave portions 54 varies in the range of 5 to 50 μm. In addition, the inner surface of each concave portion 54 is shaped like a part of a single sphere, and an inclination angle thereof is set in the range of −18° to +18°.
The term “depth of a concave portion” used herein means the distance between the substrate surface of a reflector and the deepest point of a concave portion, and the term “pitch between adjacent concave portions” used herein means the distance between the central points of adjacent concave portions, which have a circular shape as seen in a plan view. A surface as used herein is essentially a flat surface that disregards the minute irregularities (e.g. relatively microscopic crevasses or projections) present in almost every physical layer. Such a flat surface includes, for example, the substrate surface in which the concave portions are non-existent or are completely filled in.
In addition, “inclination angle” means an angle of a tangential line at an arbitrary point on the inner surface of the concave portions 54 relative to the substrate surface in a specific vertical section.
The reflector 51 has reflection characteristics similar to those of a comparative example (see FIG. 6), which will be described below. FIG. 6 is a graph showing the reflection characteristics in the case in which an incidence angle is 30°, where the vertical axis shows reflectance (reflection intensity) and the horizontal axis shows a reflection angle.
With reference to FIG. 10, an incidence angle is defined as an angle ω0 between the normal H of the reflector 51 (substrate surface) and incident light J. In addition, a reflection angle is defined as an angle ω between the normal H and reflection light K on a plane including the normal H and the incident light J. In addition, a specular reflection angle relative to the substrate surface is defined as an angle at which the incidence angle ω0 and the reflection angle ω are the same.
As shown in FIG. 6, for a specular reflection angle of 30°, the reflector 51 has a relatively good reflectance in the range of 15°≦ω≦45°.
The above-described reflector 51 of the known art exhibits relatively good reflectance over a relatively wide angle range due to the concave portions. However, as shown FIG. 6, reflectance at 30°, which is the specular reflection angle, is relatively low compared with two peaks at 15° and 45°. Accordingly, reflection characteristics of the reflector 51 are such that although relatively good reflectance is ensured for a relatively wide range, brightness is reduced in the specular reflection angle.
However, when display units installed in devices such as notebook computers, desk calculators, watches, etc., are viewed, the direction of a light source (incidence angle) and a viewing angle of a user who receives reflection light (reflection angle) are normally in a specific range. Accordingly, it would be more convenient for the user to provide a display which not only appears bright in a wide area but also exhibits especially high reflection intensity in a specific direction.
In addition, in the case in which the above-described reflector, which appears bright over a wide viewing area, is used in liquid crystal displays having a backlight, a problem exists in that light emitted from the backlight is diffused too widely at the surface of the reflector and light emitted in the specular reflection angle, which is the angle at which a user normally views the display, is reduced.